1. Field of the Invention
The invention relates to fuel injectors for internal combustion engines and, particularly, to a needle valve assembly that forms a part of the injector.
2. Background Art
It is known practice in the design of fuel injectors for internal combustion engines, particularly diesel fuel engines, to provide an injector for each of multiple cylinders of the engines. The injector includes an injector needle valve assembly that receives pressurized fluid from a fuel injector pump, the pump in turn being driven by the engine camshaft whereby fuel injection pulses are delivered to the combustion chamber of the internal combustion engine cylinders for each engine cycle, i.e., a four-stroke engine cycle. The camshaft, which is driven at one-half engine speed, develops a pressure pulse during the injection phase of the four-stroke engine cycle.
A fuel injection plunger is driven by a cam follower that engages a cam surface on the engine camshaft. The plunger and its associated fuel cylinder define a fuel pumping chamber. A control valve assembly delivers fuel to the fuel chamber. The fuel supply for the control valve assembly is a low pressure fuel pump, which circulates fluid through the control valve at intervals in the engine cycle when the control valve is in an open position. When the valve is in a closed position, direct fluid communication between the fuel pump and the fuel pressure chamber is interrupted as the plunger is stroked during a fuel injection event. The timing of the opening and closure of the control valve is controlled by a solenoid actuator under the control of an electronic engine control system.
The fuel pressure chamber communicates with an injection nozzle orifice valve that registers with an injection orifice. Pressure developed in the fuel pressure chamber acts on a differential area on the needle valve to shift the needle valve to an open position during the injection event. Movement of the needle valve under pressure is opposed by a needle valve spring that normally tends to keep the needle valve closed.
The needle valve spring is situated in a spring cage, which forms a part of the fuel injector assembly. The needle valve has a stem that defines a guide surface.
In a conventional injector assembly, a small amount of fuel may leak back across the guide surface toward the spring cage as a pressure pulse is developed by the plunger. The fluid that is leaked toward the spring cage tends to pressurize the spring cage. To avoid a hydraulic lock of the needle due to a pressure buildup in the spring cage, the nozzle spring cage typically is vented to a low pressure region of the injector. The low pressure region communicates with the fuel supply, which is under much lower pressure than the injection pressure.
Because of the continuous venting of the spring cage, pressure fluctuation may occur in the vicinity of the spring cage during operation of a conventional fuel injector. This may result in fuel vapor formation as the needle valve is advanced to an injector orifice closing position. This creates a potential for cavitation to occur in the vicinity of the needle valve spring. After extended use, cavitation damage may occur due to the unfavorable low pressure condition within the cage due to needle valve movement. The damage caused by cavitation can result in a corrosive effect on the valve spring, which in turn may cause the spring to lose its calibrated spring characteristics. This in turn can adversely affect the characteristic shape of the pressure vs. timing function fuel injection pulses.
Examples of fuel injectors of known design may be seen by referring to U.S. Pat. Nos. 5,954,487 and 6,276,610, which are owned by the assignee of the present invention.
The fuel injector of the present invention includes a needle valve that controls distribution of fuel under pressure to the combustion chamber of an internal combustion engine, such as a diesel engine. A needle valve is urged by a needle valve spring to a fuel delivery orifice closing position. A valve stem, which defines a needle valve guide surface, is situated in a needle valve opening that includes a counterbore region in fluid communication with the spring cage when the needle valve is in a closed position.
When the injector plunger is stroked, pressure built up in the pressurized fuel pumping chamber is distributed to a differential area on the needle valve, thereby lifting the needle valve against the opposing force of the needle valve spring. When the needle valve is in its fully opened position, a sealing shoulder on the needle valve seals the spring cage, thereby maintaining an increased pressure developed in the spring cage as a result of the sudden volume reduction as the needle opens. The sealing shoulder is engageable with the spring cage to trap pressurized fuel in the spring cage during a fuel injection event.
When the needle valve, at the end of the fuel injection interval, moves to the orifice closing position, the spring cage is unsealed. The spring cage is depressurized at that time by fluid distributed through a fuel vent hole in the needle valve housing. The rate of pressure decay is controlled by appropriately sizing the fuel vent hole, the vent hole in turn communicating with low pressure regions of the injector. A sudden change to a negative pressure within the spring cavity due to movement of the needle valve to its valve closing position is avoided by this controlled pressure decay. A complete bleed-down of the trapped pressure in the spring cage will occur prior to the next injection event.
Because of the residual pressure maintained in the spring cage, fuel vapor is prevented from forming and the deleterious effect of cavitation in the spring cage is eliminated.
The presence of a positive pressure in the spring cage, furthermore, is beneficial because it provides an incremental closing force on the needle valve. In a conventional design, the only force acting on the needle valve to urge it to an orifice closing position is a spring force, which opposes the pressure in the fuel pumping chamber. This incremental closing force results in a higher initial closing pressure, which will reduce the closing time for the needle valve and establish a quicker termination of the fuel injection event. The rate of fuel delivery thus is more precisely controlled than in the case of a conventional injector design.